The self-destruct sequence in this episode goes:
Kirk: Destruct sequence 1, Code 1-1-A.
Spock: Destruct sequence 2, Code 1-1-A-2-B.
Scott: Destruct sequence 3, Code 1-B-2-B-3.
After verified and engaged by the main computer, it awaits the final code for 30 second countdown, which is provided by Kirk as Code 0-0-0-Destruct-0.
The self-destruct code would eventually be reused in Star Trek III: The Search for Spock by Kirk, Scotty, and Chekov to destroy the Enterprise to prevent Klingon capture.